Matrix digestion

Determining volume fraction, by matrix digestion involves digestion of the matrix by a suitable liquid which does not attack the reinforcing fibres. Depending on the resin, digestion of the matrix can be achieved using concentrated nitric acid, an aqueous mixture of sulphuric acid and hydrogen peroxide, or a mixture of ethylene glycol and potassium hydroxide. Epoxies can generally be digested using any of these acids. BMIs, polyimides and thermoplastics usually require a sulphuric/hydrogen peroxide solution to digest the matrix. For composites fabricated from aramid fibres, the third solution must be used to prevent digestion of the fibre as well. For composites that have additives such as rubber toughened systems, care must be taken not to dissolve the additive as well.

The is calculated by measuring the weight of the remaining fibres and calculating the knowing the density of the fibres and the weight of the original sample. Accuracy of this method is determined by the accuracy of the weight and density measurements of the composite and the undigested fibre.